(i) Field of the Invention
The present invention relates to an epoxy resin and, more particularly, to an epoxy resin used for cap seal of a hard disc spindle motor.
(ii) Description of the Prior Art
With the increase of storage capacity in recent years, track density and linear density of hard disc drives have increased as well. Clean spindle motors that do not influence writing or reading of media discs have been sought for use with hard disc drive. For example, when a gas is evolved from constituent members of the spindle motor, the gas accumulates on a contact between a recording head and a recording media to cause a read error. Therefore, members with low outgassing potential have been sought as the respective members constituting the spindle motor.
The rotational speed of the hard disc drive has gradually become higher so as to improve the writing and reading rates of data. More specifically hard disc drives with rotational speeds of 5400 rpm has been replaced by those with rotational speeds of 7200 rpm, and further 10000 rpm. Accordingly, clean members constituting the spindle motor causing no change in chemical and physical properties at high rotational speed have been particularly sought.
FIG. 1 shows a typical construction (plan view) of a hard disc drive, which is composed of a spindle motor 10, a media disc 11 mounted to the spindle motor, a head 12 for writing and reading information on the media disc, an actuator arm 13 for supporting the head, a pivot 14 serving as a rotating shaft of the actuator arm, and a voice coil motor 15 for moving the actuator arm.
FIG. 2 is a schematic view of the spindle motor in which a portion is cut away to show a sectional view. The spindle motor 10 comprises a stator 30 as a fixed portion and a rotor 20 as a rotating portion. The stator 30 comprises a base 31, a shaft 32 and a stator coil 33, while the rotor 20 comprises a hub 21 mounted rotatably to the shaft 32 through a bearing 22, and a magnet 23 mounted to the hub, the magnet facing to the stator coil. In order to prevent powders, possibly produced by contact between the bearing and shaft, and bearing oil from directly scattering thereby to contaminate the media disc, a cap 26 is fixed on a top surface of the hub 21 to cover the bearing, using a cap sealant 27. The media disc 11, which is fixed to a fixture for mounting disc (not shown), is mounted to a flange surface 24 using a tap hole 25 for screwing.
As the cap sealant 27, for example, two-pack epoxy resin adhesive, UV cationic epoxy resin adhesive and thermosetting epoxy resin adhesive are commonly used. However, the two-pack epoxy resin adhesive is inferior in workability because it takes a long time for mixing before use. The UV cationic epoxy resin adhesive is difficult to cure at the non-irradiated portion, leaving insufficiently cured adhesive. The fluidity of the thermosetting epoxy resin adhesive increases with the increase of the temperature at the beginning of the curing, resulting in poor workability.
Therefore, a one-pack type UV theremosetting epoxy resin composition comprising an epoxy resin, an epoxy resin curing agent, an acrylic monomer and a photopolymerization initiator has recently been used. Consequently, it is possible to satisfy both of the workability and seal strength by coating an uncured resin around a cap, fixing by irradiating the resin with light to obtain a certain strength, and properly curing by heating.
However, it has been found that an epoxy resin adhesive of a conventional composition is not satisfactory as a cap sealant of a spindle motor for high-speed rotation, where a cleaner environment is required due to the increase of the rotational speed, e.g. 5400 rpm, 7200 rpm and 10000 rpm, because outgassing of low molecular materials occurs frequently at these higher rotational speed. According to the present inventor's study, the epoxy resin is not sufficiently cured and the uncured component may leach from within by an influence of vibration due to high-speed rotation.
In light of these problems, it would be desirable to provide a epoxy resin composition that was completed and, portions of which do not leach from within the cured adhesive.